Theft deterrent-devices which are connected to an electrical appliance and which activate an alarm when the appliance is moved are known. Examples of such devices may be found in the patent literature, for example, WO 98/06072 and WO 99/08244, each of which suffer from the disadvantage that the security device is an add-on to the appliance and is therefore removable or disarmable by a skilled burglar. Other devices are complex to install or disarm in the event that movement of the appliance is authorised (WO 98/1809).
Deterrent devices which operate to render an appliance inoperable when the power supply is removed are to be found in U.S. Pat. Nos. 4,494,114 Kaish and 5,767,771 Lamont, and in European Publication 127258 Witehira. Kaish and Witehira do not allow the necessary setting and resetting operations to be performed simultaneously on a group of appliances connected to a common power source, thereby requiring such operations to be performed independently on each appliance. Lamont has the distinct disadvantage of rendering the appliance permanently disabled except by replacement of the disabled component, which involves serious cost consequences for the owner of the appliance, in the event that the appliance is removed or in the event that the defined events occur inadvertently. More particularly, none of the devices detailed above satisfactorily address the effects of mains power failure on their operation.